The most common and most likely answer to this crossword clue is the 9 letter word OPERCULUM
I've seen this crossword clue in Financial Times
O
P
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R
C
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Buddy explains!
The straight part
We know that "Covering for slit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case covering for slit = operculum.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does OPERCULUM mean?
OPERCULUM noun- A covering flap in animals, such as a gill cover.
- The lidlike portion of a moss sporangium or of a fruit that detaches to allow the dispersal of spores or seeds.
- A gum flap covering (part of) a partially erupted tooth, usually a wisdom tooth.
- A structure which serves as a cover or lid.